This invention relates to electrosensitive recording blanks, and in particular, recording elements which respond to the action of electric currents or potentials applied thereto as, for example, telefacsimile signals and the like.
In the facsimile recording art, two types of recording blanks are well known, namely, a photographic type and the direct recording type. The photographic blank suffers from the disadvantage that the recording machine must be operated under darkroom conditions and comparatively expensive photographic developing equipment and operations are required before a complete record is obtained. However, the direct recording blank is capable of use under normal light conditions, and the record is immediately and permanently produced as the electric signals are being received.
One kind of direct recording blank that has been found universally useful contains a backing, or web of paper, which has a masking coating applied thereto, which coating is removeable in localized areas in response to applied electrical signals, so as to expose the contrasting undersurface which is usually black. In such blanks, the electrical characteristics of the blank, or substrate, material are of extreme importance. Therefore, in one type of substrate the paper backing is itself rendered electrically conductive to a predetermined degree by incorporating in the paper itself a conductive material such as powdered carbon or carbon black. In another well known type of blank, the paper backing can be of any kind of paper and has its surface treated or coated with a conducting material, generally powdered carbon, to impart the necessary blackness and conductivity.
In both the aforementioned types of electrosensitive elements, the black undersurface is masked by the white overcoating. Since it is the masking coating which responds to the applied electric signals, by way of removal, and the recording is effected by the electric stylus acting on successive elemental areas of same, it is apparent that the electrical characteristics of the masking coating are of great importance if satisfactory recordings are to be achieved.
Generally, a masking coating derives its contrasting hue with the black undersurface by having incorporated therein a conventional white pigment such as titanium oxide, zinc oxide, and the like, together with a resin binder such as a cellulose ester or ether. While such prior electrosensitive elements have been found satisfactory for most uses, they have in many cases suffered from the disadvantage that, in order to achieve the desired electric properties for the masking coating, it has been necessary to apply an extremely light masking coating. Due to the thinness of the masking layer, the blank appears gray in color instead of having a true whitish hue. The result is that when the masking coating is removed by the applied electric signals, the contrast between the unremoved areas and the black undersurface is not the most desirable. This deficient contrast may even render the record unreadable at very low levels of illumination. In addition, since the pigment in the masking layer consists of discreet physical particles, any microscopic spaces or pores between the individual particles detracts from the total masking affect and may decrease the total whitish appearance of the coat.
In order to overcome some of the aforementioned deficiencies with regard to pigmented masking layers in electrosensitive marking blanks and achieve a maximum masking affect, it was necessary to employ a masking coat which was not below a certain minimum thickness. Since the masking coat is the sensitive layer, in that it must be removed or disintegrated by the electric discharges applied thereto, either in the form of a minute arc, spark, or corona discharge, the necessary increase in thickness to achieve the desired masking quality is contrary to the marking sensitivity of the electrosensitive element to the recording signals.
In yet another alternative to the difficulties presented by a pigmented masking layer, U.S. Pat. No. 3,511,700 to Miro discloses a recording blank having a masking layer comprising a blush coating including a resin binder and a pigment. The combination of blushing and the white pigment afforded a masking layer which maintained the contrast between the recorded and unrecorded areas in the electrosensitive marking element. While this particular electrosensitive element provides desirable white masking layer and acceptable tonal response, there remains a continuing need to improve the thickness of the masking layer as well as its whiteness.
Yet another difficultly encountered with electrosensitive or electroresistive marking elements is the problem of smoke formation and ash residue. During the facsimile process the electric discharge or recording current is usually applied by means of a needle pointed electrically energized stylus. During the existence of the recording discharge or current, the masking coating is disintegrated or removed by a quasi-explosive action. To achieve the required sensitivity of recording, therefore, a certain minimum amount of electric power must be used to achieve the desired explosion or disintegration of the masking coating beneath the stylus. During the discharge process resulting in the disintegration of the coating material, smoke, as well as combustion residue, is engendered. While the ash and smoke formation may have been of no consequence in facsimile processes of the past, present day high-speed telecommunications copiers using the facsimile process require marking elements devoid of these particular difficulties.
As can be ascertained from the above, there is a continuing need for electrosensitive marking elements which provide optimum electrical response and tonal contrast and are usable in high-speed facsimile machines.